October 25, 2013

More Than 1,000 Exoplanets Have Been Discovered Since 1992

The search for extra-terrestrial planets has been ongoing for at least the past 21 years. Since the early days of exoplanet hunting, the number of such planets identified by scientists has pushed beyond a thousand, thanks largely to the advent of high-tech space observatories, such as the Kepler Space Telescope.

Exoplanets, or alien planets, are those planets that exist outside of our Solar System. So far, scientists have discovered 1,028 confirmed exoplanets, with only 12 of these alien worlds orbiting in a region considered to be “habitable.” In order to qualify as a habitable planet, an exoplanet must be at the perfect distance from its star where it is not too hot and not too cold for liquid water to exist on the surface.

The Extrasolar Planets Encyclopedia is an online catalogue of exoplanets that have been confirmed. Scientists can use the catalogue to find out about an exoplanet’s estimated mass, radius and orbiting period. Abel Mendez Torres, of the Planetary Habitability Laboratory, said that the milestone of 1,000 confirmed exoplanets was reached earlier this week after 21 years of discoveries.

Not all exoplanet catalogues list the same numbers of confirmed exoplanets, because each depends on different criteria. NASA’s Exoplanet Archive lists just 919 confirmed planets, but over 3,500 exoplanet candidates are waiting for confirmation.

According to Torres, the first confirmed exoplanets were discovered by the Arecibo Observatory in 1992, when two small planets were found amidst the remnants of a supernova explosion known as a pulsar. The planets were part of the surviving cores of former planets or newly formed bodies from the ashes of a dead star.

“This was followed by the discovery of exoplanets around sun-like stars in 1995 and the beginning of a new era of exoplanet hunting,” Torres said. “Exoplanet discoveries have been full of surprises from the outset. Nobody expected exoplanets around the remnants of a dead star (i.e. PSR 1257+12), nor Jupiter-size orbiting close to their stars (i.e. 51 Pegasi). We also know today of stellar systems packed with exoplanets (i.e. Kepler-11), around binary stars (i.e. Kepler-16), and with many potentially habitable exoplanets (i.e. Gliese 667C).”

Exoplanets range anywhere from planets that lie in the hot zone, such as “Hot Mercurians,” to those gas giants that sit in the cold zone, like “Cold Jovians.” Nearly half of the exoplanets discovered are considered “Hot Jovians,” which are planets with a close orbit to their sun and are similar in size to the planet Jupiter.

“The discovery of many worlds around others stars is a great achievement of science and technology. The work of scientists and engineers from many countries were necessary to achieve this difficult milestone,” Torres said. “However, one thousand exoplanets in two decades is still a small fraction of those expected from the billions of stars in our galaxy. The next big goal is to better understand their properties, while detecting many new ones.”

Abel said that his favorite candidate for a habitable exoplanet is Gliese 581d, which is only 20 light years from Earth. The planet has a mass of about seven Earths, with a radius about double Earth’s. Gliese 581d orbits a red dwarf star with an orbital period of only 67 days; but, because this class of star is so much cooler than even our sun it means this exoplanet still sits in the habitable zone.

“In our own solar system, Mars is cold and dry today because it was too small to retain a thick atmosphere, internal heating, and a magnetic field. However, if Mars had been a Super-Earth like Gliese 581d, it would surely still be heated from inside, have kept its magnetic field and thick atmosphere, and would likely still have liquid oceans (and perhaps even life!) on its surface,” Torres said.